The prior art embraces capping machines consisting in a carousel rotatable about a vertical axis and comprising a drum that carries a plurality of capping units equispaced angularly about the periphery, each comprising a hollow shaft aligned on a vertical axis, of which a bottom portion incorporates a sliding bush and is fixed to a capping head equipped with a gripper mechanism.
The carousel also carries a disc located beneath and coaxial with the drum, affording seats on which to stand the containers.
Each seat receives a single container at an infeed station. At the same moment, the corresponding capping unit receives a cap.
As the carousel rotates, between the aforementioned infeed station and an outfeed station, the cap will be placed by the capping unit on the neck of the container and screwed tight.
The capping unit, and consequently the hollow shaft, is capable of axial motion, necessary in order to position the cap on the neck of the container, and rotary motion, necessary in order to screw the cap onto the container.
Conventionally, the rotary motion of the single capping unit is induced by a system of gears set in rotation by a central gear wheel aligned concentrically on the axis of the carousel.
The axial motion of the capping unit, on the other hand, is induced by a cam-following roller attached to the aforementioned bottom portion of the hollow shaft and engaging the track presented by a fixed cam of drum type, coaxial with the carousel.
The cap is held and twisted onto the container by a gripper of which the movements are induced mechanically, through the agency of means located inside the capping head and comprising calibrated springs such as will ensure a firm hold on the cap.
The gripper jaws are spread apart on completion of the screwing step by the action of cam means associated with the gripper and able to overcome the resistance offered by the springs.
Also associated with each capping head is a respective mechanical clutch that serves to shut off the transmission of rotary motion to the capping head, once the cap has been screwed tight and at the moment when the gripper jaws are spread.
Capping machines of the type in question present certain drawbacks.
Being invested with rotary and axial motion as described above, the hollow shafts that carry the capping heads will tend to labour under the forces of the gripper tension springs and of the cam-following rollers. In addition, the hollow shafts are subject to high levels of friction attributable to the use of sliding bushes.
Moreover, the single capping units are comparatively heavy and consequently become subject, during the rotation of the carousel, to appreciable centrifugal and inertia-related forces.
These various factors dictate the need to utilize high strength materials and to adopt precision machining procedures, in particular for the cams, also to follow extremely accurate and complex assembly procedures, in order to achieve and maintain a correct axial alignment of each capping unit with the axis of the relative container positioned beneath, and to ensure that this same axial alignment is checked periodically.
In addition, the system by which rotation is transmitted to each capping unit in prior art machines, utilizing an epicyclic train, is extremely complex and costly.